Conventionally, there is known a sensor device for transmitting/receiving radio waves to sense e.g. An object or human body using a Doppler signal (see, e.g., Patent Literature 1). Such a sensor device senses a sensing target based on the amplitude value of the signal depending on the moving velocity of the sensing target or the distance between the sensing target and the sensor device.
Japanese Unexamined Patent Publication No. 2004-286675 discloses a sensor device comprising an oscillator for generating a transmission wave to be emitted outside, an antenna part for transmitting the transmission wave and receiving reflection waves reflected by a sensing target as reception waves, a mixer part for generating output signals obtained by interference of the reception waves, and a determination part for determining the presence or absence of an object based on the output signals obtained by the mixer part. In this sensor device, the mixer part generates two output signals having a phase difference of 90 degrees. The determination part determines the presence or absence of an object based on the amount of change of the two output signals with respect to an arbitrary reference value.
In such a conventional sensor device, the two output signals are provided with a phase difference of 90 degrees. Thus, the sensing signal has a generally unique value for the distance from the sensor device to the sensing target. Accordingly, the stationary or generally stationary state of the object can be sensed accurately. Furthermore, distortion in the output signals can also be lowered while keeping the phase difference of the two output signals at 90 degrees. Thus, approaching/leaving of the sensing target can be determined accurately from the temporal relation of the two output signals on the time axis.
Here, the transmission wave transmitted from the sensor device and the reception wave reflected by the sensing target are undulate. Thus, the two output signals generated based on the transmission wave and the reception wave are also formed undulated. Accordingly, a location with a small amount of change of the two output signals, i.e., a location with a lowered sensing accuracy, is formed depending on the distance from the sensor device to the sensing target.
The aforementioned sensor device in Japanese Unexamined Patent Publication No. 2004-286675 determines the presence or absence of an object based on the amount of change of the two output signals provided with a phase difference of 90 degrees. Thus, when the sensing target is in the stationary or generally stationary state at the location with a small amount of change of the two output signals, the sensing accuracy may be lowered. This causes concern about lowering the determination accuracy of the object in the determination part.